Pneumatic-hydraulic system for operating well pumping equipment



Aug. 14, 1951 s, v, SMITH 2,564,285

PNEUMATIC'HYDRAULIC SYSTEM FOR OPERATING WELL PUMPING EQUIPMENT Filed March 11, 1948 4 Sheets-Sheet l /w /e/ we Jam? L Juli- IN VEN TOR.

ArroPAz-r Aug. 14, 1951 s,'v SMITH 2,564,285

PNEUMATIC HYDRAULIC SYSTEM FOR OPERATING WELL PUMPING EQUIPMENT Filed March 11, 1948 4 Sheets-Sheet 2 ZN? 7/ ll 673 60 6 7 WW 1 3622 ix M Aug. 14, 1951 s, v, sM|TH 2,564,285 7 PNEUMATIC-HYDRAULIC SYSTEM F OR OPERATING WELL PUMPING EQUIPMENT Filed March 11, 1948 4 Sheets-Sheet 3 I INVENTOR- Jauvzz 146M171? Patented Aug. 14, 1951 'PNEUMATICFHYTDRAULIC SYSTEM FOR P- EB ATING fUMPINfi EQUIPMENT .Samu l V- Sm th, lfBeMhi Qalif- I Application March 11, 1948551311241 No. 14,355. 14:01am. (01; .su-sry fluid pressu e i o mu ic ed to newe the Jack p Partic lar c nt mi lat sd :i a s stem wh r y the jack pis on is powe d b pressurizsd gas and hydraulic fluid a manner involving utilization. of the energy of the piston vdown stroke ;b y compression of the a for v5 1 1 110atiori to thepistonpn its up-stroke. One vof m ajo ob t s to provi e a tem :of ,high operating efficiency made possible by powering the jack'piston with aseous and liquid fluids under conditionsisuch that operation of the p ston. t erma :b i ;19ad, can be accomplished under-w applied hydraulic pressures with correspondingly low energy input required. to pressurize the hydraulic fluid, For application of both gas and liquid pressures to, actuate the load, the invention contemplates a nor/{e1 and efiicient jack piston and cylinder assembly so constructed to permit separate application and maintenance of the two fluids a ainst the piston, and specifically in a manner wheneby the pressurized :gas acts 9 Hi y a ain t the piston in an elevating direct on, while the hydraulic fluid pressure is alternately applied to the piston in opposite directions to eflect ;re.- cipr-ocation. As will appear, by maintaining proper relationship of the piston ar as and appl ed as p e) c p o a ion o :Vh ipi tQnand' its .load may be accomplished by low hydraulic fluid pressure. I

v Aiurther object of-the ingven-tionis to ,mediiythe speed of the piston :to --the advantage {the pump, preferably bya unit hav ingthe .duaLfimea tions of accommodatin surges .lof the hydr fluid .betwe nit p ssurizingsourc nd the jack piston, and of delivering to the pressurized air supply. whatever ompr ssed air that may J3? required for lmake ilp as by reason 01? slight :air le kages from the system. F r thi pu pose I employ a fluid pressure displaceable demise, such as an .assiiia y sister and cy inder ransected to the jack c linder ydraulic sy tem in a ma nsr suah thattt pistes is d snlaceahle by as sure a s .ed by he draul i id i on; 91: pis on..- 53 that i s tra el smqd ed in adamant-.1: with th -s e of iquid into and ou -p the auxil a y eylin ,tne sccns auent displacemen o the aus i y n tourne b used emp es air to the pr s u ized r sup y t e pres ure dr ps new s .limi

pni m la d by he ,inrent qn is a. novel p essure entral. 91 th hydra c s ste onswims t9 nr r a the hydrau ic flu d p essure from s xeeedins the m x sn m the central is set teammate-t n firefl uch esulatiqn is efiectesi preferably by ma nta in an -pass sec ion of th h d aul c s stem. a .se table and Ya abl re ease re ulator aqantrql cd b #RIQS" sure taken rom :therrs surized ,g s supnl As w app under teumsta sss whi h might oth wise rssu t vsrleas as .o njury or t qu pme t as consequence of excessive .hydraniis Pressu e intro-ease the fla -pass con .1 wi l rims.-

n sm o h! id, reliably to 11' It th pr ssure inc e se t :w. ster e sl d -f a nrs tic has s 911s a onal f atures and -ism a l of .wh h eacher wit the the or-see s w ll Joe-unden- U 4 d an 'to b s .asirantase rrom theis law ns deta ed des ri t on o a t picale baaimea 9 theisr tim shown lay the accompanying drawings, in which:

- Eis 2 451 gene a ew showin in e1 vation 1 Jaras embly includin i s p wer source, and. the relationship Well head.

is viswsh dss diagrammatically stariusrp Q 'F e by" al i s i systems, and he a s siation w th th rziston and cylinder asssm l appe rins s amen;

. isi an en arged vi il ustratin mor :in d tai t s t ir 0 the piston and yl nder s embly; 7

F gs- 4 an 5 ar irasme tar enlarged seci s i u t atin he s sins aenfin ent withininner and outer portions of the piston tructure;

5 is a :Yifiw il ustrating diagrammatically 11 hsYdi ffi dlic quntrel D l0;t and master valves, tag in w th theiiiuidfiw-imsscpntrol;

if the unitto the Fig. '7 is a view showing the air check and make-up cylinder in longitudinal section; and

Fig. 8 is an enlarged section of the fluid bypass control means.

Referring first to the general showing of Fig. l, the equipment comprises the jack piston and cylinder assembly I8 positioned above the well casing II and operating to vertically reciprocate the pump liquid being discharged from the casing head I3 through connected line I4. The jack piston in the assembly I8 is operated, as will later appear, by gas and hydraulic fluid pressures supplied from the sources contained inthe general assemblyin'dicated at 15, all of which together with the jack cylinder at H] may be mounted as a unit, on a base or skid structure I69 for movevalve 54 alternately through lines 55 and 56 respectively into spaces 2'! and 33 to reciprocate the piston assembly by exertion of the hydraulic fluid pressure against the piston faces 58 and 59. At this point it may be mentioned that preferably these piston faces will correspond closely in area. Referring to Fig. 6, the master valve 54 comprises a cylinder 69 containing a floating piston 6| including heads 62. 63, 64 and 65 on the rod 66. Connecting with the cylinder are the return branches 61 and 68 leading through line 69 to the reservoir 59.

Operation of the master valve 54, and specifically displacement of the floating piston 6|, is governed by a pilot valve mechanism 19 which in turn is controlled by the travel of the piston assembly 23. The latter carries a rotatable arm II to which is attached a depending rod I2 ex- 8 tending through the guide I3, and carrying arm mediate section I9 and an inner section 29, the

last named sections being open at their upper ends and spaced annularly and concentrically within the outer sectionJ'I, as illustrated. The

TH and clamp 2'12 secured to the rod and seat- 1 ing within the head opening 28 of the piston. Thus the rod extends concentrically through and in balanced relationship to the piston and cylinder assemblies. As best illustrated in Fig. 3 the cylinder head I8 contains seal rings 39 retained. by rings 3| and 32 and engaging the outer surface of the piston section to prevent fluid leakage fromthe head and what may be referred to asa third space 33 between the piston and the outer cylinder wall I'I.

The outer piston-section 24' has a head 33I carrying seal rings 34 and 35 engagingthe cylinder walls I1 and I9. As shown in Fig. the seal rings 35, which -may be of continuous annular cast iron or other metal composition and containing non-metallic seal inserts 36, are held in place by the retaining-rings 31 and 38 in the relationship illustrated. The inner piston section 26 similarly has a head 39 carrying seal ring assemblies 49 and 4| similar to the rings 34, 35, rings 4| being held in place by 'theretaining rings 42 and 43.

The pressure fluid supply system I5 includes a compressed 'air'reservoir 45 through "which air pressure is constantly communicated to space to apply against the piston face 46, by way of line 47 containing a hand valve 48. The reservoir initially may be charged with air under pressure approaching or corresponding to the air pressure to be maintained against the piston face 46, from any suitable source, as by way of the-valve inlet 49. Thereafter, the 'air pressure required for operation of this system 'mayfbe maintained in the reservoir 45 by utilizing the energy developed by the piston assembly 23, as

will later appear. 1 I

A hydraulic fluid, such as mineral oil, is sup-' plied from reservoir 59 through line 5| to pump 53 driven by motor 54, see Fig. Land is discharged by the pump under control of the master I4. The latter is engageable with projections actuating positions. Rod TI carries a cam bracket I8 engaged by a follower 19 attached to the pilot valve rod 89 having a reduced section 8| within cylinder 82 at opposite sides of a valve or piston portion 83. Cylinder'82 is connected with opposite ends of the master valve cylinder 69 by way of lines 84 and 85, as shown in Fig. 6. Branches 86 and 81 lead from cylinder 82 through the return line 88 to the reservoir 59.

Line 5| is communicable at the discharge side of the pump 53 with line 69 by way of by-pass line 89 containing a valve 99 which may be of any suitable type responsive to the fluid pressure, and specifically to air pressure communicated from the reservoir 45. As illustrated, the valve 99 is shown to comprise'the valve 9| proper having a stem 92 attached to piston 93 within cylinder 94 to which air pressure is communicated from the reservoir by way of line 95 containing a manually settable valve 96, see Fig. 2. Adjustment of valve 96 imposes controllable air pressure in cylinder 94 and therefore air pressure against piston 93 to maintain such resistance to opening ofthe valve 9| under the influence of the hydraulic pressure communicated from the pump through line 89, as will limit the maximum hydraulic pressure transmittable through lines 55' and 56 to the piston and cylinder assembly I9. Accordingly, by selective setting of the bypass control it is possible to confine the maximum hydraulic pressure as desired and within safe limits;

Lines 55 and 56 connect respectively through branches 98 and 99 with the auxiliary hydraulic fluid accommodating assembly, generally indicated at I99, the details of which appear in Fig. 7. The assembly I99 comprises in effect a pair of individual surge receiving units which may be incorporated in a single cylinder [9| containing a central partition I92 through which extends the rod I93 interconnecting the heads I94 of a floating piston P. 'The cylinder heads contain compression chambers I96 to which air is admitted'past check valves I91. The cylinder heads also contain the outlets I98 closed against return air flow by check valves I99, communicating through branches III! and Il9a with line III leading to the air reservoir 45." Line I I contains a pressure regulating valve, conventionally indicated at I I2 which may be set to maintain a controllable air pressure in thereservoir, as required for the selected pressure to be exerted against the jack piston.

assets 8.

In considering the operation of the system, assume the parts to bein the positions illustrated inFigs. 2 to "7, with the piston assembly 23at -the IimitJcif its down stroke. Pressurized hydraulic fluid is delivered from line 5 I. through the master valveeylinder between pistons 63 and 54, through line 55 into the cylinder space 21. The hydraulic fluid thus applied to. the piston face 58 supple--v merits the air pressure communicated to space 25 and acting against the piston face it, to elevate the piston assembly. At this point it may be observed. that ordinarily the air pressure acting against the piston may rather closely approach that pressure required to elevate the piston and its lead taken through the rod I 2, so that the magnitude of hydraulic fluid pressure required for operation of the piston is relatively small.

- Preferably the air ipressurewill be just under,

say within a range" of .25 lbs, that pressure required to elevate the piston and its lead... The supplementary hydraulic fluid pressure in chamber 27 thus operates to elevate the piston assem bly and the rod string throughout its pumping stroke..

During elevation "of the piston, hydraulic fluid isd isplaced from space 3-3 through line 56 and the master valve cylinder between the piston,

valve heads 54 and 65, and through lines 68 and Bil'to the reservoir 50. At the same time, the pilot valve is positioned to communicate the hydrauliepump discharged pressure through line I cylinder 82 at the left of the'valve pistonllfi, and line 851w the master valve chamber lid-to maintain the master valve in its Fig; 6 position. During the previous displacement of the master valve [H from right to left, the fluid in chamber H6 will have been displaced through line 84, the pilot valve cylinder, and lines it and 88 to the reservoireo.

As 'the piston assembly v23 approachesthe upper limit of its stroke, the valve control arm it engages projection: 76 to raise rod TI and its cam lsfshifti'ngthe pilot'valve to bring its piston section 83 to the dotted line position 83a, whereupon the pump discharge pressure is communicated through line 84 to the the master valve cylinder shifting valve 61 to bring its intermediate piston sections 'to the positions indicated by the dotted lines etid-and 64a. .As a result, th'e'high pressure hydraulic'fluid is discharged through line 5% into space 33 to applyagainst the piston face 59 during the down stroke of the'piston assembly 23. Throughout the piston down stroke, fluid in space 27 is returned through line 55, the master valve cylinder and lines 61 and B9 to the reservoir 58. the master valve shifts to the'right, fluid .is displaced from chamber H5 through line 85, the pilot Valve'cy1ind'er'82 and lines 8'! and 88 to the reservoir 5!]. From the foregoing itwill be apparent that the relationships between the hydraulic and the air'pressures, and the areas of the piston 23'against which such pressures act, may besu'ch that the weight of the pum'p rod' l2 and its lead is suflicient to move the piston throughout its down stroke. Should the load encounter such resistance, as by reason of the required force to be transmitted to the well pump, or for any other reason, that the load weight is insufficient topowe'r the piston assembly throughout its down stroke, the supplemental hydraulic pressure act ing aga'inst thepist'on face 59 will supply the'ener gyfrequired for the piston down stroke.

It is anticipated that under some circumstances upward or downward movement of the load I 2 might encounter such resistance ;as,to

cause damageor'breakage of therod string, or other partsunder the influence of the full hydraulic power of which the pump 53 is capable of developing; By proper setting of. the bypass control valve 9H,.aspreviously explained, the effective hydraulic pressure transmittable to the piston and cylinder assembly in may be. kept within safe but adequate operating limits The function of the assembly I00 is two fold :in that it serves to modify the speed of the piston assembly. 23, and also to compress and deliver to the reservoir 45 such make-up air as the system may require from time to time as for example because of slight leakages. As the piston assembly 23 starts its upward travel, its speed in relation to the rate of oil displacement by the pump 53uis retarded due to the fact that the pressure in lines 55 and 98 is sufficiently high tozdisplace the piston P toward the right and permit a portion of the oil to enter the enlarging chamber 2| 5. Then as 'the jack piston gains upward speed, the "pressure against piston head i M at the right of the air previously compressed during movement of the piston to the right, tends to move the latter toward the left and displace the oilfrom chamber 216 to the jack cylinder chamber 21, supplementing the pump delivery rate. During the reciprocations of piston P, air may be compressed through lines I ID or IlOa to the reservoir 45 if its pressure has fallen below that which regulator H2 is set to maintain. Upon reversal of the travel of the piston assembly 23, and starting its down stroke, the pressurized -oil is transferred through lines .56 and 99 to the auxiliary unit chamber Eli. Initially in the down stroke, the oil pressure may shift the piston .P toward the'left from an intermediate position .due to a greater pressure which at-afi-rst is required to overcome the piston inertia, .following which the pistonmay return to an. intermediate position after the piston is in motion and less oil pressure is required to keep it in motion. Thereafter, the tendencyof the oil pressure. will be to increase (due to compression of the an applied to the piston) :and shift the position .to the left, permitting an inflow of oil tochamber and consequent retardation of the jack piston speed during the latter extent of its down stroke.

Referringxagain to Fig. 1, it is contemplated that all workingcparts possibly subject to hydraulic fluid leakage, may be contained within a single chamber serving as a common housing and receptacle, for such leakage. Accordingly,

. the reservoir 50, pilot .and master valves 10 and 54, the check cylinder I00 and by-pass control valve 90, all may be contained within a tank 2I'5 mounted together with the remaining parts 'of the described pump jack assembly on' the skidso shifting the jack unit is shown typically to comprise a cylinder 1 l8 stationarily secured to the base .l'Gl, and containing a piston H9 attached at l 20'to the support or skid I52. pressure fluid from line 5! is delivered through line I26 to a manually reversible 4-way valve High i I23 from which li'nes I24 and I25 run to opposite ends of the cylinder II8. Return flow line I2! and its branches I22 connect the valve I23 with the hydraulic pump suction line 69.

After disconnection of the assembly I!) from the pump rod string I2, the 4-way valve I23 may be thrown to a position causing the high pressure hydraulic fluid to be delivered to cylinder IIB through line I24, causing the skid support I62 and the unit carried thereby, to be shifted to the right a distance such that the assembly IIIv clears the well head. During such displacement of the unit, the opposite end of cylinder H8 is in communication with pipe 69 through lines I25 and I2I. Upon reversal of the valve I23, high pressure fluid is delivered through line I25 to cylinder H8 to return the jack unit to its Fig. 1 position over the well, while .fluid return from the left end of cylinder H8 occurs through lines I24 and IN. I

I claim: Y

LA well pump jack comprising a vertically extending cylinder assembly including three concentric and annularly spaced tubular sections containing inner and router spaces, a tubular piston assembly comprising an outer tubular section reciprocable within said outer space and an attached inner section reciprocable within said inner space, means for supporting a pump rod or the like by the piston, and means for operating the piston by fluid pressure.

2. A well pump jack comprising a vertically extending cylinder assembly including three concentric and annularly spaced tubular sections containing inner and outer spaces, a tubular piston, assembly comprising I an outer tubular section reciprocable within said outer space and an attached inner section reciprocable within said inner space, means for supporting a pump rod or the like by the top portion of the piston so that the rod extends downwardly through said inner cylinder section, and means for operating the piston by fluid pressure.

3, A well pump jack comprising a vertically extending cylinder assembly including three concentric and annularly spaced tubular sections containing inner and outer spaces, a tubular piston assembly comprising an outer tubular section reciprocable within said outer space and an attached inner section reciprocable within said inner space, there being a third space between said outer cylinder and piston sections, means for supporting a pump rod or the like by the piston, and means for communicating fluid pressure to said spaces to operate the piston.

4. A well pump jack comprising a vertically extending cylinder assembly including three concentric and annularly spaced tubular sections containing inner and outer spaces, a tubular piston assembly comprising an outer tubular section reciprocable within said outer space and an attached inner section reciprocable within said inner space, there being a third space between said outer cylinder andpiston sections, means for supporting a pump rod or the like by the piston, means for constantly communicating gas pressure to said outer space and against the outer piston section tending to raise the piston assembly, means for communicating to said inner space hydraulic pressure acting against inner piston section and tending to raise the piston assembly, and means for communicating to said third space hydraulic fluid pressure acting against said outer piston section and tending to lower the piston assembly.

5. A well pump jack comprising a vertically extending cylinder assembly includingthree concentric and annularly spaced tubular. sections containing inner and outer spaces, a tubular piston assembly comprising an outer tubular sec-. tion reciprocable within said outer space and an attached inner section reciprocable within said inner space, there being a third space ,between said outer cylinder and piston sections, means for supporting a pump rod or the like by the piston, means for constantly communicating gas pressure to said outer space and against the outer piston section tending to raise the piston assembly, means for communicating to said inner space hydraulic pressure acting against inner piston section and tending to raise the piston assembly, and means for communicating to said third space hydraulic fluid pressure acting against said outer piston section and tending to lower the piston assembly, the hydraulic fluid pressure acting against substantially equal effective areas of said inner and outer .piston sections.

6. A well pump jack comprising a vertically, extending cylinder assembly including three concentric and annularly spaced tubular sections containing inner and outer space-s, a tubular piston assembly comprising an outer tubular section reciprocable within said outer space and an attached inner section reciprocable withinsaid inner space, there being a third space between s'aid outer cylinder and piston sections, means for supporting a pump rod or the like by the piston, means for constantly communicating gas pressure to said outer space and against the outer piston section tending to raise the piston assembly, means for communicating to said inner space hydraulic pressure acting against said inner piston section and tending to raise the piston assembly, means for communicating to said third space hydraulic fluid pressure acting against said outer piston section and tending to lower the piston assembly, a pressurized hydraulic fluid source, and valve means controlled by movement of said piston assembly to transmit the hydraulic fluid pressure from said source alternately to said inner and third spaces.

'7. A well pump jack comprising a vertically extending cylinder assembly including three concentric and annularly spaced tubular sections, containing inner and outer spaces, a tubular piston assembly comprising an outer tubular section reciprocable within said outer space and an attached inner section reciprocable within said inner space, there being athird space between said outer cylinder and piston sections, means for supporting a pump rod or the like by the piston, means for constantly communicating gas pressure to said outer space and against the outer piston section tending to raise the piston assembly, means for communicating to said inner space hydraulic pressure acting against inner piston section and tending to raise the piston assembly, means for communicating to said third space hydraulic fluid pressure acting against said outer piston section and tending to lower the piston assembly, a pressurized hydraulic fluid source, a pilot valve, means for actuating said pilot valve in accordance with the piston assembly travel, a master valve'responsive to movement of said pilot valve, and means whereby movements of the master valve transmit the hydraulic fluid pressure from said source alternately to said inner and third spaces.

8.A" -well pump jack comprising a vertically extending cylinder assembly including three concentric-and annularly spaced tubular sections Containing inner and outer spaces, a tubular piston assembly comprising an'outer tubular section reciprocable within said outer space and an attached inner section reciprocable within said inner space, ihere being a thirdspacebetwee'n said outer cylinder and piston sections, means for supporting'a pump rod or thelike by the piston, means for constantly communicating gas pressureto said outer space and against the outer piston section tending to raise the piston assembly, means for communicating to said inner space hydraulic pressure acting against inner piston section and tending to raise the piston assembly, means for communicating to said third space hydraulic fluid pressure acting against said outer piston section and tending to lower the piston assembly, a pressurized hydraulic fluid source, valve means controlled by movement of said piston assembly to transmit the hydraulic fluid pressure from said source alternately to said inner and third spaces, a reservoir charged with compressed air and in constant communication with said outer space, and a pump actuated by the energy of said piston assembly and operable to compress air into said reservoir.

9. A well pump jack comprising a vertically extending cylinder assembly including three concentric and annularly spaced tubular sections containing inner and outer spaces, a tubular piston assembly comprising an outer tubular section reciprocable within said outer space and an attached inner section reciprocable within said inner space, there being a third space between said outer cylinder and piston sections, means for supporting a pump rod or the like by the piston, means for constantly communicating gas pressure to said outer space and against the outer piston section tending to raise the piston assembly, means for communicating to said inner space hydraulic pressure acting against inner piston section and tending to raise the piston assembly, means for communicating to said third space hydraulic fluid pressure acting against said outer piston section and tending to lower the piston assembly, a pressurized hydraulic fluid source, valve means controlled by movement of said piston assembly to transmit the hydraulic fluid pressure from said source alternately to said inner and third spaces, a reservoir charged with compressed air and in constant communication with said outer space, a cylinder, having an air inlet and connected to said reservoir, a piston in the last mentioned cylinder, and means for actuating said piston by said hydraulic fluid under pressure created by said piston assembly to compress air into said reservoir.

10. A well pump jack comprising a vertically extending cylinder assembly including three concentric and annularly spaced tubular sections containing inner and outer spaces, a tubular piston assembly comprising an outer tubular section reciprocable within said outer space and an attached inner section reciprocable within said inner space, there being a third space between said outer cylinder and piston sections, means for supporting a pump rod or the like by the piston, means for constantly communicating gas pressure to said outer space and against the outer piston section tending to raise the piston assembly, means for communicating to said inner 'space' hydraulic pressure "acting against inner piston section and tending to" raise the piston assembly, means for communicating to said third space hydraulic fluid pressure acting against said outer piston section and tending to lower th'e'piston assembly; a pressurizedi'hydraulio fluid source, valve means 'controlledby movement of said piston assembly; to transmit the hydraulic fluid pressure from said source alternately to said inner and third spaces,"a;com pressed air jsource communicablefl with said outer sp ce/and means controllable by'rthe pressure of tpecompressed air to limitthe maximum bydr zlic pressure transmitted from said hydraulic fluid source to said inner and third spaces.

11. A well pump jack comprising a piston reciprocable to raise and lower its load'in the well, a compressed gas source, a pressurized hydraulic fluid source independent of said compressed gas source, means communicating the compressed gas pressure against the piston in a load elevating direction during both strokes of the piston, means communicating the hydraulic fluid pressure to the piston, means for alternately applying the hydraulic fluid pressure to the piston in advancing directions during both of its strokes, and a valve device controlled'by and in accordance with differential pressures of the gas and hydraulic fluid and operating to limit the maximum hydraulic pressure applied to the piston.

12. A well pump jack comprising a vertically extending cylinder and a piston therein reciprocable vertically to raise and lower the jack load in the well, a compressed gas source, a pressurized hydraulic fluid source independent of said compressed gas source, means communicating, the compressed gas pressure to a first surface of the piston to aid in elevating the load, means communicating the hydraulic fluid pressure to second and third surfaces of the piston, means for alternately applying the hydraulic fluid pressure to said second and third surfaces to reciprocate the piston, and a valve device controlled by and in accordance with diiferential pressures of the gas and hydraulic fluid and operating to limit the maximum hydraulic pressure applied to the piston.

13. A well pump jack comprising a vertically extending cylinder assembly including a plurality of annularly spaced concentric tubular sections, a piston assembly including a plurality of annularly spaced concentric tubular sections reciprocable vertically between said cylinder sections, means for constantly applying fluid pressure to a first downwardly facing area of the piston assembly, means for alternately applying fluid pressure to second and third upwardly and downwardly facing areas of the piston assembly, and means for connecting the piston assembly with a pump rod.

14. A well pump jack comprising a vertically extending cylinder assembly including a plurality of annularly spaced concentric tubular sections, a piston assembly including a plurality of annularly spaced concentric tubular sections reciprocable vertically between said cylinder sections, means for constantly applying fluid pressure to a first downwardly facing area of the piston assembly, a pressure fluid source, valve means controlling the delivery of fluid from said source to the piston and cylinder assembly, means for actuating said valve means in accordance with the piston assembly travel to alternately apply fluid pressure to second and third upwardly and downwardly facing areas of the piston assembly, and means for connecting the piston assembly with a pump rod.

SAMUEL V. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Number Number Name Date Hubbard Sept. 27, 1932 Trinks Mar. 21, 1933 Lower Oct.29, 1935 Christensen 'Apr. 22, 1941 Kyle et a1. July 27, 1943 Twyman et a1 Apr. 25,1944 Buchet June 13, 1944 Ross Dec. 4, 1945 Ross Jan. 28, 1947 FOREIGN PATENTS ,Country Date Great Britain May 16, 1938 

